Hello, everyone. What I have to present is contrary to most of modern biological thought, but from it, it would seem that universal common descent is a paradox that is not possible under natural conditions. The vast majority of readers might prejudge it as wrong immediately, but I respectfully ask that people give it a chance without presupposition and with an open mind. Even if you disagree, I certainly appreciate anyone who takes the time to read this and consider the thought experiment. By the end of it, you will probably, at the very least, be struggling to come up with a reconciliation.

Let's begin by examining feline penises. They all have a common attribute, and that is the fact that they are all barbed with numerous, small spikes that are of a similar substance to a fingernail. The barbs on the penis provide the required physical stimulation for ovulation in cat females. It is also thought by some that the penis-barbs also serve the purpose of scraping out competing sperm.

Natural selection would explain that, while in early development, a certain primitive feline of the male sex received a genetic mutation which resulted in a barbed penis. That male feline had sex with multiple females, and the barbs on the penis scraped out competing sperm, causing the cat to have more offspring than other males. The children retained the barbed-penis gene and afterward the males matured and had sex with other females, passing on their genes more than males of their generation without barbs, and the process continued with exponential growth until all male felines had barbed penises.

However, there's a problem with that. The female cat's reproductive system relies upon the spines of the male's penis in the first place. Unlike most mammals, cats have a specific physical trigger for ovulation, and that trigger is the spines. That ultimately means that, not only did the initial male cat have to randomly receive the mutation of the barbed penis, but a female in the same general area in a similar generation would also have to randomly received the genetic mutation that makes them precisely receptive to barbs of the other cat's general size and composure in order to ovulate. That by itself is almost impossible in terms of probability, but that's not all. The exponential notion of how the barbed penis would have spread goes out the window given this, because only the one female who happened to have the mutation would be receptive to the barbed penis. That means that only the couple's offspring would have the male and female reproductive qualities of felines as we know them today, and they would have had to have literally be in a bizarre situation such as hiding underground while all other felines on the planet are destroyed by an asteroid, and repopulated their species through extensive incest. This entire set of criteria is so unlikely to occur naturally, it is far within reason to call it practically impossible.

Yet, we're presented with the compelling image of a barbed penis which scrapes out competing sperm as a testament to how virtually all traits common to a species are here because of natural selection. It makes sense unless you factor in a detail like the female's reproductive system. This type of logic can be explained by the diagram below:

The claim positions itself to support two known facts, then uses itself to support a new, wider conclusion. Note that this particular claim, as explained above, does not hold up to reason. While the facts in black are observably true, the claims in red make sense given the facts presented, but are proven wrong by examining them in relationship to other factors.

There is really no way for the barbed penis and the female reproductive system that relies entirely upon the barbed penis to have developed separately and have overtaken the population of early felines who did not have barb-receptive reproductive systems exponentially through natural selection. Mathematically, the only reasonable conclusion is that felines were, at one time, in an initial state in which both sexes had barb-compatible reproductive systems. Extending the line of logic of pairing barbed and non-barbed reproductive systems, how could a species of animal with barbed penises have ultimately evolved from a previous ancestor that did not have barbed penises? Conversely, how could a species with non-barbed penises have evolved from a species with barbed penises?In a hypothetical situation in which an "ancestor" species has a non-barb-receptive reproductive system, one individual would have to have either the barbed or barbed-receptive mutation, depending on sex, but the odds of another member of that species being nearby and having the mutation that would compliment theirs are so ridiculous, they are beyond practically impossible.This is the Keyhole Paradox, because an oversimplified way of explaining it is to imagine that, by random chance, both a key and the exact-fitting lock for the key would have to randomly appear through mutation, and besides the incredible unlikelihood for this mutation to occur, all other animals of the initial species would be incompatible with either the "key" or the "lock," save for the offspring of this unlikely pair.This logically disproves the claim that felines resulted from an ancestor that did not have barbed penises. Consequently, this also logically disproves the idea of universal common descent, regardless of species. Because of this keyhole paradox and how sexual reproduction works, it's practically impossible for a species with a barb-oriented sexual reproduction system to have evolved from a species whose sexual reproduction method is not barb-oriented through natural selection.

Complementary mutations do not need to arise at the same time. If the barbs give the male an advantage, that does not require females to only be receptive to males with barbs as well; there is an advantage to the male even without that.

At any time after the males obtained their barbs, then there was no longer any disadvantage if some mutation prevented females from being able to be fertilized by a non-barbed male - because there were no non-barbed males around any more.

Alternatively, if barbed males had an advantage, there might be positive selective pressure towards females only being receptive to barbed males, because then the females would ensure they were only mating with males with higher genetic quality.

If you have a less "oversimplified" way of trying to explain your paradox, I think you should try and produce it. You also seem to be suggesting that the only line of reasoning supporting universal common descent is the one that you show in your picture, so you may want to address that as well.

Also, an argument about improbability cannot by its nature constitute "disproof" of anything. Improbability and impossibility are two different things. Even if your argument were correct, there is nothing preventing complementary traits from simultaneously arising in the same species at some frequency anyways. This does not require "killing off" the rest of the species - so long as they could still outbreed, the traits would still be maintained.

irtree wrote:Consequently, this also logically disproves the idea of universal common descent, regardless of species. Because of this keyhole paradox and how sexual reproduction works, it's practically impossible for a species with a barb-oriented sexual reproduction system to have evolved from a species whose sexual reproduction method is not barb-oriented through natural selection.

Your argument, if it were to succeed, seems to be against evolutionary novelty in sexually reproducing organisms. Assuming that you dont dispute the evolution of sexually reproducing organisms from non-sexually reproducing ones, I dont see how you've offered an argument against a universal common ancestor. That ancestor need only be a non-sexually reproducing organism which, at different stages in its own evolution, evolved into different sexually reproducing organisms. On the other hand, if you hold that sexually reproducing organisms didn't evolved from non-sexually reproducing ones, then you are stuck with the fact that there are sexually reproducing organisms which have sexually dimorphous complementary features, despite your argument. So, in addition to the points made by AstraSequi, as your conclusion is apparently false by observation, your argument must either be pathological, or suffer from false premises or faulty reasoning.

I see AstraSequi's point: the barbed penis could have provided an advantage enough to eventually replace the non-barbed-penis, the mutation of the barb-receptivity could have arisen some time after, and the barb-receptivity could have provided enough of an advantage to proliferate throughout the entire population, as well.

The problem lies in the incompatibility factors between the current reproductive system and the one that, based on the reproductive systems of most mammals, would have existed in the hypothetical state before this reproductive system made its two-stage proliferation to encompass all felines. The model you suggest could work theoretically for minor reproductive changes, but not for something which is as incompatible as cat reproduction. Imagine that you are on a romantic honeymoon with your husband, and you eventually engage in your first act of intercourse. Things go fairly well, until you feel 150 1-millimeter-long "fingernails" pricking and digging into your vaginal walls, causing you to scream for help, collapse backwards, and scream obscenities as you try to understand what just happened and reel in fear of what damage might have been caused.My argument is that each reproductive system's barbed or barb-ready state is simply not compatible with the non-barb-receptive/non-barbed state. On one hand, if you have a barbed penis attempting copulation with a non-barbed vagina, you have a whole host of issues that are not present in non-barbed/non-barbed-compatible copulation, for obvious reasons. Whereas, although there are some apparent cuts, it induces ovulation in barbed/barbed-ready copulations, penetration by a barbed penis into an unadapted vagina would be marred by a whole host of damaging factors, including uncontrolled bleeding which dilutes the sperm, the mate immediately reacting to the perceived threat of this unusual source of pain, the permanent damage this would cause (to what extent, I can't conjecture, but there could very well be some), just to name a few.A sudden mutation of having barbs on a mammal's penis isn't very likely to cause it to be very successful in the mating department, much less put it in a situation where it has an advantage in reproduction and the trait becomes common to the entire species through natural selection. On the other hand, there is absolutely no possibility of the barb-receptive trait being passed on before the barbed-penis trait, because it specifically requires contact with the barbs for ovulation.

Ughaibu's argument is something I thought about after posting this. What about evolving from an asexual organism with the barbed penis and barbed-receptive female reproductive system already in place, much in the same regard as non-barb-involving systems are in place in other sexually reproducing organisms. The chief problem with that is, according to modern evolutionary theory, both felines and various organisms in the order Carnivora, many of which do not have barb-involving reproductive systems, evolved from a common sexually-reproducing organism, Miacoidea. Thus, the argument at the end of my initial message still applies. According to current evolutionary thought, felines did not evolve from a different asexual organism than dogs or bears or weasels. In fact, rabbits and camelids are the only other known species of mammal which also induce ovulation through mechanical means during copulation, and according to modern thought, they did not descend from Miacoidea at all.

I realize that impossibility and improbability are not the same thing, which is why I used the term "practically impossible." If something has, for instance, a 0.000000000000001% chance of being true with all data considered, it's not valid enough to accept as a fact. Sure, there's still a "possibility" there, but not in any realistic sense.

irtree wrote:The problem lies in the incompatibility factors between the current reproductive system and the one that, based on the reproductive systems of most mammals, would have existed in the hypothetical state before this reproductive system made its two-stage proliferation to encompass all felines. The model you suggest could work theoretically for minor reproductive changes, but not for something which is as incompatible as cat reproduction.

There is no reason the system was just as incompatible when it first appeared. It could be that the first barbs were very small and soft, and this could still have given an advantage. After the females had adapted to this, then the barbs made another small change - and so forth.

Also, pain/injury does not necessarily reflect reproductive success. Even if you are horribly injured from intercourse, but still survive long enough to give birth and care for your children, and then the person with the barbs goes and has a thousand other children as well because of whatever advantage he gets from them, then the adaptation will still spread. (Look up "traumatic insemination" on Wikipedia, which is even worse!)

Ughaibu's argument...

So what is your position? You seem to understand natural selection and common ancestry; which specific kinds of evolutionary changes do you say are impossible?

My impression is that you say that two adaptations cannot arise if they are complementary, which I think means that the animal could survive with having neither or both but not if it had only one.

Is that correct? However, if so, it doesn't imply anything about universal common descent, only that natural selection is unable to take certain paths.

I realize that impossibility and improbability are not the same thing, which is why I used the term "practically impossible." If something has, for instance, a 0.000000000000001% chance of being true with all data considered, it's not valid enough to accept as a fact. Sure, there's still a "possibility" there, but not in any realistic sense.

I agree. However, you will have to apply a statistical correction (Bonferroni or similar). That is to say, if something has a 1 in a million chance of happening in a given year, but then you wait 10 million years, then it will probably happen about 10 times. Or if the population grows to have 10 times as many animals during that time period (=10 times the chance of any particular occurrence happening), then it will probably happen about 100 times.

That is, the probability itself is important, but so is the number of chances you get. This was one of the hardest things for me to understand when I was learning about evolution - the truly massive amount of time involved. If you flip a coin enough times, eventually you will get ten heads in a row.

You also haven't produced your "less oversimplified" version of the paradox, at least from my interpretation of your post.